The invention relates generally to wind turbine generators and more specifically to a system and method for integrated fault and personnel protection system for wind turbine power systems providing output to a load.
Generally, wind turbines use the wind to generate electricity. The wind turns multiple blades connected to a rotor. The spin of the blades caused by the wind spins a shaft of the rotor, which connects to a generator that generates electricity. Specifically, the rotor is mounted within a housing or nacelle, which is positioned on top of a truss or tubular tower, which may be as high as about 100 meters. Utility grade wind turbines (e.g., wind turbines designed to provide electrical power to a utility grid) can have large rotors (e.g., 30 or more meters in diameter). Blades on these rotors transform wind energy into a rotational torque or force that drives one or more generators, rotationally coupled to the rotor through a gearbox. The gearbox may be used to step up the inherently low rotational speed of the turbine rotor for the generator to efficiently convert mechanical energy to electrical energy, which is provided to a utility grid. Some turbines utilize generators that are directly coupled to the rotor without using a gearbox. Various types of generators may be used in these wind turbines.
Many devices, such as wind turbines, include power converter systems. A power converter system is typically used to convert an input voltage, which may be fixed frequency alternating current, variable frequency alternating current, or direct current, to a desired output frequency and voltage level. A converter system usually includes several power semiconductor switches such as insulated gate bipolar transistors (IGBTs), integrated gate commutated thyristors (IGCTs or GCTs), or metal-oxide semiconductor field effect transistors (MOSFETs) that are switched at certain frequencies to generate the desired converter output voltage and frequency. The converter output voltage is then provided to various loads. Loads as used herein are intended to broadly include motors, power grids, and resistive loads, for example.
To accommodate the need for greater power from windfarms, individual wind turbine generators are increasingly being provided with higher power output capability. To accommodate the higher power output from the wind turbine generators, some wind turbine systems are provided with multiple parallel converters (also known as converter threads). Multiple parallel converters may also provide an advantage in wind converters due to the desire for high availability and low distortion
Typically, power converter systems use multiple power converter bridges in parallel with gating control to expand power-handling capability. In wind turbine applications, a power converter bridge usually refers to a three-phase converter circuit with six power switches. In order to meet both grid side and machine side power quality requirements, such systems generally use large and costly filters to smooth out pulse width modulated waveforms. Such systems sometimes cause overheating of the generator and/or transformers and other distortion-sensitive equipment due to high harmonic components, when the large and costly filters are minimized.
FIG. 1 is a block diagram of a typical power system employing multiple parallel converters. Wind turbine power system 10 is configured for supplying power to a load 21, which may be an electric power grid. A generator source 14 is configured to generate an AC input power. The AC input power is provided to power converter system 20. The power converter system 20 comprises converters 20-1 through 20-N. The converters are coupled in parallel and configured to receive the AC input power from the generator source 14. The power converter system 20 is configured to convert the AC input power to an AC output power. The AC output power is provided to load 21.
For cost and size reasons, each thread is connected to a common point on the grid and the plant with conductors that are usually sized in accordance with the rating of each thread and not the system rating.
Converter control system 24 is configured to provide control signals for the operation of the power converter system 20. The converter control system is coupled to the converter system and is configured to drive the converter system according to predesignated switching patterns. The predesignated switching patterns provided by the converter control system may provide for synchronous gating of the multiple parallel converters or may provide an interleaved manner of control for each converter thread with phase displaced gating signals to reduce overall switching harmonic components due to cancellation of phase shifted switching waveforms.
FIG. 2 is a block diagram of one typical thread of a power converter system. Wind turbine embodiments, for example, typically comprise three-phase power converter systems. Converter 20-1 represents one thread of power converter system 20. Converter thread 20-1 comprises generator-side converter bridge 30 for AC-DC conversion, DC link 35, and load-side converter bridge 40 for DC-AC conversion to a suitable voltage and frequency. Generator converter bridge 30 may be implemented using six semiconductor power switches 45. Similarly, load-side bridge 40 may be implemented using six semiconductor power switches 45. Generator-side chokes 50 and load-side chokes 55 may be sized to enable either non-interleaved or interleaved gating.
Switching of the power semiconductor switches in the converter threads causes a difference in voltage between the parallel converters, which creates a common mode current that flows between the converter threads, even without having a ground fault on the system. Operating pulse width modulated converters in shifted modulation phase can be made to produce similar power conducting currents while reducing the low frequency distortion current when the two converters are connected to the generator or grid through combining reactors. This allows a reduction of additional distortion reduction equipment, like passive filters, insulation and conductors to be reduced. However, while the net (result of the combination of converters) distortion current is reduced, the individual converter circulating current is worsened.
A common mode current will flow in a circular loop between the power converter threads, but not have any impact on the net current in either the grid or the plant. Common mode chokes 60 suppresses the high frequency (switching frequency range) common mode cross current that links both generator-side converters and the load-side converters.
FIG. 3 illustrates common mode current flow in a power system converter with n-paralleled converter threads (20-1 to 20-n) connected to a grid 21 and to a wind turbine generator 14. For example, it is possible that a current can flow into thread T1_L_Ia 110 and out T1_G_Ia 115 and return through thread T2_G_Ia 120 and T2_L_Ia 125. There are many combinations of loops for such current that will not affect the net current. However, these common mode currents, as well as normal mode circulating currents, force converter switching devices and other components to operate closer to thermal limits.
Further, these common mode currents may cause a direct error in the measurement of ground fault currents of that loop, thereby making fault detection more difficult. Normally, the sum of phase currents for galvanically isolated three phase subsystems should be zero. However, for a subsystem, such as an individual converter thread, the common connection of the individual converter threads on the generator-side and on the load-side can result in a non-zero summation of currents due to common mode currents flowing between threads.
Not only is the system unlike the more normal three-phase system, in that the distortion current higher, but it is also unlike the three phase system in that neither the summation of the phase grid side current, nor the generator-side current be assured to near zero when the grid side and the generator-side currents are combined through reactors without bulky additional isolated windings in transformers and/or generator.
Fault protection based on detection of currents differentiated for “normal” three phase sine waves has to deal with high distortion and non-zero summation of three phase current. Yet, protection of individual three phase converter threads requires rapid fault detection, isolation and repair.
FIG. 4 illustrates a typical wind turbine power system 10 with a power converter system 20 for utility power applications, including a three-phase generator-side converter for AC-DC conversion and a three-phase load (or grid) side converter to invert DC-AC, with a DC link interposed. Three-phase AC power, generated by the wind turbine generator 14 at the top of the tower 13, is connected along the height of the tower through tower cables 15 to power conversion equipment and controls, which may generally positioned towards lower levels 12 of the tower 13, in one or more power plant machinery (PPM) levels.
Multiple parallel converter threads 20-1 to 20-4 are provided to accommodate the power conversion requirements of the wind turbine generator 14. A typical converter thread 20-1 may include, a generator-side (AC-DC) converter 30, the DC link 35 and a load-side (DC-AC) converter 40, with accompanying charging circuits 27. On the generator-side of these converter elements, a common mode filter 60 may be provided to minimize common mode currents circulating between the individual converter threads. A generator-side filter 50 and a load-side filter 55 may be provided to reduce distortion of the voltage waveforms on the respective side of the converter. Surrounding these elements, line contactor 60 may provide control for isolating the converter threads. Further, fuse protection 70 may be provided on the ends of the converter thread 20-1, generally in physical proximity to the converter thread 20-1. Converter threads 20-2, 20-3 and 20-4 are similarly arranged.
The converter threads 20-1 to 20-4 may be connected on the load-side to a manually operated circuit breaker 75 located, for example, in electrical distribution panel 80 in proximity to the base of the wind turbine tower. The manually operated circuit breaker 75 may provide isolation from grid side power and capability for lockout-tagout at the electrical distribution panel 80 during maintenance. The manually operated circuit breaker 75 may also provide overcurrent trip capability. The manually operated circuit breaker 75 may be connected on the load-side to a main transformer 85 for transforming the converter power output to a common bus voltage for the windfarm in which the wind turbine is located. An isolation circuit breaker 90 for the wind turbine power system 10 may also be provided in the base of the wind tower for isolation of the wind tower from grid side power with overcurrent trip capability and lockout-tagout capability.
The typical structure for a wind turbine electric power output as previously described in FIG. 4 suffers from a number of deficiencies including: 1) lack of circuit protection for the tower cables 15 from the wind turbine generator source 5; 2) the generator-side converter cable entry may be subject to arc flash hazard, has no circuit protection from the wind turbine generator source 5, and lock-out tagout provisions for protection from the generator source 5 must be exercised by climbing the tower to apply a brake; 3) contactors within the converter do not supply isolation for the converter; 4) fuse protection for the load-side converter is potentially subject to a high ionized arc during fault conditions that may interfere with fault interruption; 5) the cable entry for the load-side converters may be subject to an arc fault hazard; 6) generator-side and load-side cable entrances to the power distribution panels may be subject to arc flash hazard, and generator-side cable size may not be coordinated with circuit protection; and 7) no ground fault circuit protection may not be adequately provided on the load-side cable entry.
Accordingly, there is a need to provide a structure and method for providing protection for wind turbine generator converter systems that will detect faults and distinguish faults under conditions of operation with common mode current and provide protection against catastrophic damage during true fault conditions and also provide personnel protection during fault conditions and maintenance.